1. Field of the Invention
The present invention relates to a pacemaker having a pulse source for delivering heart stimulation pulses to at least one heart stimulation electrode.
2. Description of the Prior Art
In biomedical therapy of heart diseases electric stimulation of the tissue is often utilized. In the presence of normal, spontaneous activity of the heart, stimulation is inhibited or synchronized to the sensed natural event.
It is important that contraction of the heart is achieved as a result of stimulation, so-called stimulation capture. When a stimulation apparatus is implanted tile minimum stimulation voltage for capture is normally determined. It is well-known that the threshold value of stimulation capture is initially temporarily increased to a maximum over a number of weeks. It is also well-known that this threshold value will vary with the passage of time. Because of these effects the stimulation level must be set rather high to insure stimulation capture. Often a stimulation voltage of twice or more the measured threshold value is selected as a standard procedure. As a result the current consumption is increased with a factor of four above the measured threshold value for stimulation capture, with a shortened longevity of the stimulator as a consequence. This is a serious inconvenience and there is continuing need within this technical field to reduce the necessary stimulation energy and to lengthen the intervals between replacement surgeries.
A larger contact area with the tissue of the stimulation electrode normally results in a more stable stimulation threshold, whereas a smaller contact area of the stimulation electrode, below about 3 mm.sup.2, causes larger spreads and variations in the threshold value, cf. eg. Stokes et al, The Mythology of Threshold Variations as a Function of Electrode Surface Area, PACE, Volume 14, November 1991, Part II, pp. 1748-1751. A small electrode contact area, however, has a higher electrical contact impedance and consequently losses of energy in electric wires and output switches are reduced. Further, the current density seems to be one factor of major importance for obtaining stimulation capture and a high current density is obtained at a lower stimulation voltage with a small electrode contact area with the tissue.
Fibrotic tissue growth and fat-cells will increase the contact impedance between the electrode and surrounding tissue and result in large local variations. This effect causes problems when using small electrode surfaces, but is equalized over larger electrode surfaces. Higher energies will also overcome this difficulty; a higher energy can stimulate cells located a distance away and will thus "bridge over" a nonconductive layer.
Implantation of a pacemaker is normally followed-up by monitoring the value of the stimulation threshold, measured in voltage peak amplitude, and stimulation impedance variations, as seen from the pacemaker. This impedance is normally of the order of 500 ohm. Larger deviations in these values indicate inappropriate pacing conditions.
Since the use of small stimulation electrodes has indisputable advantages, the interest in such electrodes has increased. Stimulation pulses are currently determined by voltage amplitude and duration, sometimes by stimulation current and duration. If the contact impedance then is increased, which normally occurs when reducing the contact area, the stimulation energy is, however, decreased with an obvious risk for loss of capture.